The present invention relates to an inkjet printing device comprising means for the image-wise application of hotmelt ink to a receiving material.
The present invention also relates to a hotmelt ink and a combination of hotmelt inks suitable for use in such an inkjet printing device.
The present invention further relates to a method of forming an image of hotmelt ink on a receiving material, wherein drops of liquid hotmelt ink are sprayed by an inkjet printhead onto a receiving material in accordance with electrical image signals fed to the inkjet printhead, and heating the hotmelt ink applied to the receiving material.
Hotmelt inks do not contain solvents to keep them in the liquid state such as are provided in water-soluble inks. Hotmelt inks are solid at room temperature and are not made liquid by heating until just before application to the receiving material. Once applied to the receiving material, the hotmelt ink sets again. U.S. Pat. No. 5,043,741 describes the problems which may occur in these conditions. If the temperature of the receiving material is too low, the ink sets too rapidly and hence too much remains on the surface of the receiving material. As a result, in addition to reduced print quality due to inadequate coverage, the adhesion to the receiving material is less satisfactory. If, on the other hand, the temperature of the receiving material is too high, the ink sets too late, so that it penetrates deeply into the receiving material, in which conditions the ink may even reach the back of the receiving material. Excessive penetration of the ink into the receiving material can lead to inadequate optical density as a result of dilution or the ink no longer being visible on the surface. In addition, too long a heating may result in undefined flowing out of the ink. In this case the fiber structure of the receiving material in particular plays a part. The ink then flows out along the locally present fibers so that an irregular form is obtained. This effect is known as "feathering".
Known devices therefore try to keep the temperature of the receiving material constant by keeping the temperature of a guide surface for the receiving material constant. In that case, however, no consideration is given to the differences in the properties of different receiving materials or the time that they remain in contact with such a guide surface. The device according to the said patent is therefore suitable for rapidly controlling the temperature of such a guide surface. For this purpose, the guide surface is continuously in heat contact with both heating means of the conventional electrical resistance heating type and cooling means of the thermoelectric type. The whole is accommodated in a practically closed housing with defined inflow and outflow air openings. The associated temperature control ensures that temperature of the guide surface for the receiving material remains between 25.degree. C. below and 25.degree. C. above that of the ink melting temperature.
One disadvantage of such a system, apart from the complexity of the temperature control, is that although the properties of the receiving material have less influence, they are still present. The heat regulation obtained as a result is not optimum so that the problem of feathering is not really prevented. In practice, feathering can still occur.
U.S. Pat. No. 5,023,111 also describes a hotmelt printing device. Here, the ink applied to the receiving material is kept above the melting temperature for some time. For this purpose, the receiving material is also guided over a heated guide surface. The latter is curved at the beginning and end in the direction of transport of the receiving material in order to counteract any curvature of the receiving material. At the end of the transport path, along the heated guide surface, a rapid temperature drop is obtained by the fact that part of the guide surface is in heat contact communication with a cooling body, locally.
The disadvantage of this is again the complex construction required, in which it is only the distortion of the receiving material that is counteracted. Adequate measures for preventing excessive or inadequate flowing out are not described. Here again feathering can still occur.
U.S. Pat. No. 4,971,408 also refers to distortion of the receiving material during application of hotmelt ink. This is attributed inter alia to moisture being withdrawn from the receiving material in the case of heating uncontrollably. Mention is also made of the problem of keeping the guide surface for the receiving material at a constant temperature. In accordance with the hotmelt printing device described in U.S. Pat. No. 4,971,408, the temperature of the receiving material is kept below the melting temperature of the ink during the ink application, whereafter the ink present on the receiving material is again heated, in controlled manner, for a period of between 0.5 and 10 seconds, to above the melting temperature in a separate re-heating device. Preferably, a heat radiator is used for the re-heating. The disadvantages of the heated guide plate are admittedly not present, but the relatively long time during which the receiving material with the ink has to be heated may result in unwanted heating of the receiving material and ink and hence again cause feathering of the hotmelt ink.
U.S. Pat. No. 4,202,618 describes a copying machine in which fixing is also effected by means of short radiation pulses originating from a flash lamp. However, this relates to an electrophotographic process wherein the inks used are of a completely different type. In an electrophotographic process a charged photo conductor is exposed image-wise whereafter non-heated toner of thermoplastic material mixed with carbon is applied to the resulting charge image. This toner image is then electrostatically transferred to receiving material. The toner on the receiving material is then exposed to short radiation pulses originating from a flash lamp. However, toner of this kind has a completely different flow behaviour. On heating, it does not become completely liquid like hotmelt ink, but only plastic. An absorption of such toner in the receiving material as in the case of hotmelt ink cannot therefore occur.